1. Field of the Invention:
The present invention is directed to a method of manufacturing an effective blast furnace coke, and more particularly to a method of manufacturing an effective blast furnace coke which contains a substantial amount of low grade coal.
2. Description of the Prior Art:
It is well known that blast furnaces, and especially large sized blast furnaces, require high quality cokes to sustain their operation. At the same time, however, due to the current global shortage of good quality coals, the price of effective blast furnace cokes has gone up. In order to compensate, the coke industry has developed a number of manufacturing processes for the production of blast furnace cokes which can utilize low grade coals, e.g., non-coking coals or poorly coking coals (these being the coals which account for most of the available coals and are the cheapest in cost, but which heretofore have not been considered suitable for use as raw material coals for the manufacture of blast furnace cokes). However, none of these processes have been found to be totally acceptable.
For example, the following manufacturing processes have been developed which make use of low grade coals: a preheated coal charging process (coaltek system or precarbon system) wherein some or all of either a coking coal or a blended coal, which consists of a coking coal and a low grade coal, is preheated at temperatures of from 200.degree. C. to 350.degree. C. and then charged into the coke oven (see Japanese Patent Publication No. 23495/46, published on July 5, 1971); a partial briquette charging process wherein briquettes containing low grade coal are added to a blended coal to be charged into the coke oven (see Japanese Patent Publication No. 7375/46, published on Feb. 24, 1971); a caking substance adding process wherein a charging coal is prepared by adding artifical coking coal or a caking substance to blended coal (see JA-OS 85803/53, laid open for public inspection on July 28, 1978); and a selective pulverization process wherein a coking coal of a kind which has its coking property segregated according to its grain size distribution is pulverized by use of a sieve (see Japanese Patent Publication No. 45763/49, published on Dec. 6, 1974 and Japanese Patent Publication No. 19321/53, published on June 20, 1978).
The above noted preheated coal charging process is believed to provide enhanced strength to the produced coke because the bulk density of the coal charged into the coke oven is increased and the spaces between adjacent coal particles are decreased, and also because the 100.degree. C. zone is totally absent or short during the vaporization of the moisture, the heating rate in the plastic zone is lowered, the thickness of the plastic layer is expanded, and the possibility of adjacent coal particles coalescing is enhanced. Although the blending ratio of the low grade coal is variable with the particular kind of coal being used, it is thought to have a limit of 20% by weight (hereinafter indicated simply in %).
In the case of the above-noted partial briquette charging process, various views have been advanced as to the mechanism which leads to the manifestation of its effect. A typical theory is that expansion of the briquettes in the coking process causes compaction of the coal surrounding the briquettes, which results in an improvement in the coking property. When the blending ratio of briquettes is 60%, incidentally, the bulk density of the coal charged in the coke oven reaches its peak and the stength of the produced coke is improved to the greatest extent. In the operation of this system on a commercial scale, however, the segregation which occurs in the briquettes in the coke oven results in difficulties in discharging the produced coke from the coke oven. Thus, the blending ratio of briquettes is said to be generally limited to about 30%. The proportion of low grade coal which can be blended in total amount of the charging coal is about 20%, though it is variable with the type of low grade coal which is used.
The above-noted caking substance adding process aims to overcome the fluidity problems and improve the quality of the produced coke by the addition of a caking substance. The quality of the caking substance, therefore, is important. Since the caking substance is generally higher in cost than the coal, the proportion of the caking substance economically desirable for addition to the charging coal is generally limited to about 10%. For this reason, the blending ratio of low grade coal is said to be about 20%.
The above-noted selective pulverization process aims to improve the coking property of the charging coal by pulverizing coking coal of a kind which has its coking property segregated according to its grain size distribution, screening the pulverized particles through a sieve 3 to 6 mm in mesh size and pulverizing again the course particles retained on the sieve, whereby the inert particles which reduce the coking property and are concentrically present in the coarse-particle zone will be uniformly distributed throughout the charging coal.
With any of the processes described above, however, the highest possible blending ratio for the low grade coal in the charging coal is about 20%. Thus, the need of developing a method capable of producing a blast furnace coke containing low grade coal in an increased blending ratio has become pressing. Research and developments, thereto, are being promoted on this subject from various angles.
It is, therefore, an object of this invention to provide a method for the manufacture of blast furnace coke by blending low grade coal with coking coal in a high blending ratio.
Another object of the present invention is to provide a method for the manufacture of blast furnace coke by the operation of the partial briquette charging system without necessitating any special means for the prevention of segregation.